index.js

/**
 * Module dependencies.
 */

var Emitter = require('emitter');
var clone = require('clone');
var type = require('type');
var ease = require('ease');

/**
 * Expose `Tween`.
 */

module.exports = Tween;

/**
 * Initialize a new `Tween` with `obj`.
 *
 * @param {Object|Array} obj
 * @api public
 */

function Tween(obj) {
  if (!(this instanceof Tween)) return new Tween(obj);
  this._from = obj;
  this.ease('linear');
  this.duration(500);
}

/**
 * Mixin emitter.
 */

Emitter(Tween.prototype);

/**
 * Reset the tween.
 *
 * @api public
 */

Tween.prototype.reset = function(){
  this.isArray = 'array' === type(this._from);
  this._curr = clone(this._from);
  this._done = false;
  this._start = Date.now();
  return this;
};

/**
 * Tween to `obj` and reset internal state.
 *
 *    tween.to({ x: 50, y: 100 })
 *
 * @param {Object|Array} obj
 * @return {Tween} self
 * @api public
 */

Tween.prototype.to = function(obj){
  this.reset();
  this._to = obj;
  return this;
};

/**
 * Set duration to `ms` [500].
 *
 * @param {Number} ms
 * @return {Tween} self
 * @api public
 */

Tween.prototype.duration = function(ms){
  this._duration = ms;
  return this;
};

/**
 * Set easing function to `fn`.
 *
 *    tween.ease('in-out-sine')
 *
 * @param {String|Function} fn
 * @return {Tween}
 * @api public
 */

Tween.prototype.ease = function(fn){
  fn = 'function' == typeof fn ? fn : ease[fn];
  if (!fn) throw new TypeError('invalid easing function');
  this._ease = fn;
  return this;
};

/**
 * Stop the tween and immediately emit "stop" and "end".
 *
 * @return {Tween}
 * @api public
 */

Tween.prototype.stop = function(){
  this.stopped = true;
  this._done = true;
  this.emit('stop');
  this.emit('end');
  return this;
};

/**
 * Perform a step.
 *
 * @return {Tween} self
 * @api private
 */

Tween.prototype.step = function(){
  if (this._done) return;

  // duration
  var duration = this._duration;
  var now = Date.now();
  var delta = now - this._start;
  var done = delta >= duration;

  // complete
  if (done) {
    this._from = this._to;
    this._update(this._to);
    this._done = true;
    this.emit('end');
    return this;
  }

  // tween
  var from = this._from;
  var to = this._to;
  var curr = this._curr;
  var fn = this._ease;
  var p = (now - this._start) / duration;
  var n = fn(p);

  // array
  if (this.isArray) {
    for (var i = 0; i < from.length; ++i) {
      curr[i] = from[i] + (to[i] - from[i]) * n;
    }

    this._update(curr);
    return this;
  }

  // objech
  for (var k in from) {
    curr[k] = from[k] + (to[k] - from[k]) * n;
  }

  this._update(curr);
  return this;
};

/**
 * Set update function to `fn` or
 * when no argument is given this performs
 * a "step".
 *
 * @param {Function} fn
 * @return {Tween} self
 * @api public
 */

Tween.prototype.update = function(fn){
  if (0 == arguments.length) return this.step();
  this._update = fn;
  return this;
};